1. Field of the Invention
This invention relates generally to the surface modification of substrates, and in particular, to a the surface modification of porous metal substrates using a cold spray of material.
2. Description of Related Art
The growing popularity of portable electronic devices has produced an increased demand for compact and correspondingly portable electrical power sources to energize these devices. Developments in robotics and other emerging technology applications are further increasing the demand for small, independent power sources. At present, storage or rechargeable batteries are typically used to provide independent electrical power sources for portable devices. However, the amount of energy that can be stored in storage or rechargeable batteries is insufficient to meet the need of certain applications.
Hydrogen/air fuel cells (H/AFCs) have enormous potential as a replacement for batteries. Because they can operate on very energy-dense fuels, fuel cell-based power supplies offer high energy-to-weight ratios compared with even state-of-the-art batteries. Fuel cells are of particular interest to the military, where significant efforts are being made to reduce the weight of power supplies that soldiers must carry to support high-tech, field-portable equipment. There is also considerable potential for utilizing fuel cell-based power supplies for commercial applications, particularly for portable applications, where small size and low weight are desirable.
A common H/AFC is a polymer electrolyte membrane (PEM) fuel cell. PEM fuel cells are constructed of an anode and a cathode separated by a polymer electrolyte membrane. Functionally, fuel cells generate electricity by reacting hydrogen with oxygen to produce water. Since oxygen can typically be obtained from the ambient atmosphere, only a source of hydrogen must be provided to operate a fuel cell. Merely providing compressed hydrogen is not always a viable option, because of the substantial volume that even a highly compressed gas occupies. Liquid hydrogen, which occupies less volume, is a cryogenic liquid, and a significant amount of energy is required to achieve the extremely low temperatures required to liquefy gaseous hydrogen. Furthermore, there are safety issues involved with the handling and storage of hydrogen in the compressed gas form or in the liquid form.
One method of producing hydrogen is by processing hydrocarbons such as methane (natural gas), propane, butane, and liquid fuels such as gasoline, diesel and JP-8 or oxygenates such as methanol. The choice of fuel and the choice of the method of processing, such as steam reforming, partial oxidation, and autothermal reforming, depends to a large extent on the type of service, such as, portable, stationary or automotive. Hydrogen can also be produced by cracking ammonia. The product stream from the fuel processor when a hydrocarbon feed is used contains hydrogen in addition to unreacted hydrocarbons, other products such as CO, CO2, and diluents such as nitrogen. In essence, the hydrogen concentration in the product stream can be in the 40 to 75 volumetric percent range depending on the type of fuel and the method of processing. Methods such as water gas shift and preferential oxidation are used to reduce the CO concentrations to acceptable levels of no more than 50 parts per million, but increase the complexity of the system.
One method of separating the hydrogen from the product stream concerns the use of hydrogen separation membranes. These membranes are in general composed of pure palladium or alloys of palladium and can either be supported or unsupported. Supports are usually porous ceramics with porosities in the 40 to 60 percent range. Commercial hydrogen separation membranes are unsupported and are tubular in nature. The separation membranes are composed of one tube or more than one tube that are bundled together and are designed to supply high purity hydrogen, that is, hydrogen which is greater than 99.9 percent pure. The separation membranes are generally limited to operating temperatures below 450° C. due to the sealing techniques used, which is inherent to tubular configurations, and are generally costly. To reduce costs, considerable efforts have been focused on supporting thin membrane layers on porous ceramic support, with porosities in the 40 to 60 percent range. The problem with porous ceramic supports is that the adherence of a metallic membrane to a nonmetallic ceramic substrate is a major problem, particularly when the supported membrane is exposed to thermal cycling between room temperature and elevated temperatures of no less than 400° C. In this case too, the ceramic supports that are evaluated are in the form of tubes, to mimic the construction and service of unsupported commercial membrane modules.
The hydrogen separation membrane, preferably made of pure palladium or palladium alloys, is deposited on the porous substrate by techniques such as electroless plating and electroplating. The substrate is exposed to the precursor metal salts of predetermined compositions in a deliberate fashion, and the salts are subsequently decomposed to yield a metal film or membrane on the support. Achieving a uniform pin-hole or crack free membrane is reportedly influenced by the surface morphology and pore size distribution of the ceramic support. In particular, a smooth substrate surface having small pores of no more than 1 micron in diameter and uniform pore size distribution are believed to be desirable. Small pores prevent wicking or seepage of the salt precursors through the pores. A smooth surface favors a uniform membrane deposition, however some level of roughness is needed to provide adhesion. The fact that these properties are possessed by ceramic substrates make them particularly attractive for experimentation and development efforts.
Relatively less focus and efforts have been directed towards the use of porous metal substrates. This is mainly because the porous sintered metals are relatively less porous than the ceramic substrates when compared at similar pore diameters, and therefore allows less gas flow for a defined pressure drop across a given substrate. For example, a porous stainless steel substrate rated as 0.2 micron grade (Mott) has a porosity of only about 7% and a Darcy value of about 0.025. To achieve a high flow rate of gas to the membrane, without a large pressure drop, a porous stainless steel substrate with a porosity of about 30% and a Darcy value of about 0.2 would be useful. However, the synthesis and sintering process during the manufacture of the metal substrates does not allow for pore size and porosity to be varied independent of one another, and to achieve a 30% porosity requires the use of 1.5-2 micron grade porous materials.
In addition, the surface of metal substrates is relatively rough. Typically, techniques used to smoothen the surface, such as shot peening, results in pore closure and consequently decreases porosity. In addition, shot peening has a deleterious effect when the substrate contains large pores of no less than 1 micron. This is because the large metal shots that are needed cause ‘pitting’ of the surface and increase surface roughness. Even exposure to ion beams, while being less destructive then shot peening, and able to provide relatively consistent pore size, do not provide optimal surface uniformity. The nominal required thickness of a membrane over a porous metal substrate, to avoid pin-holes, gaps and leaks, is directly related to both the pores size and the surface uniformity.
In spite of the above negative characteristics, porous metal substrate based membranes represent a desirable feature for integration with a fuel processor for PEM cell applications. They have higher thermal conductivities than ceramics and therefore result in high heat transfer rates. They can be sealed in a relatively straightforward fashion using welding to components such as reactors, unlike ceramic substrates that need grafoil or soft metal gaskets. Inherently, the metal to metal adhesion of the membrane to the substrate can be expected to be better than that expected using metal-ceramic substrates.
Therefore, it is a desideratum to produce a porous metal substrate with a more uniform surface to receive a membrane.